The properties of structural metallic parts depend both on the composition of the alloy and on the fmal microstructure of the fabricated parts. The microstructure, in turn, depends both on the alloy system and on the conditions of its solidification. The interaction of alloy and process determines the microstructural features, such as type and morphology of precipitates, grain size, distribution and location of shrinkage microporosity, which greatly affect the properties of the structural parts. Thus, magnesium alloy parts produced by die casting exhibit very different properties from those produced by sand, permanent mould and other casting methods. It is the task of the alloys designer to interfere with the microstructure of the processed parts and try to optimize it in order to improve the final properties.
A comprehensive analysis of literature data and the inventors' experience show that there are few potential directions for developing cost-competitive Mg die castable alloys with improved creep properties.
The inexpensive die cast alloys having a Mg matrix and containing aluminum and up to 1% zinc (AZ alloys) or aluminum and magnesium without zinc (AM alloys) seem to offer the best combination of strength, castability ans corrosion resistance. They have however the handicaps of poor creep resistance and poor high temperature strength, especially in cast parts. The microstructure of these alloys is characterized by Mg.sub.17 Al.sub.12 intermetallic precipitates (.beta.-phase) in a matrix solid solution of Mg--Al--Zn. The intermetallic .beta.-phase compound has a cubic crystal structure incoherent with the hexagonal close-packed structure of the matrix solid solution. Besides, it has a low melting point (462.degree. C.) and can readily soften and coarsen with temperature due to accelerated difffusion, whereby it weakens the grain boundaries at elevated temperatures. It has been determined to be the key factor that accounts for the low creep resistance of these alloys. In die cast parts, the microstructure is further characterized by a very fine grain size and a massive grain boundary area available for easy creep deformation.
When developing Mg alloys for die casting applications, it should be taken into consideration that the presence of Al in the alloy is strictly required to provide good fluidity properties (castability). Hence, a magnesium alloy should contain a sufficient amount of Al in the liquid state prior to solidification. On the other hand, the presence of Al leads to the formation of eutectic Mg.sub.17 Al.sub.12 intermetallic compounds--the aforesaid .beta.-phase compound--which adversely affect creep resistance. Hence, it wouild be desirable to suppress its formation by the introduction into the alloy of a third element, generically indicated herein as "Me", that can form an Al.sub.z Me.sub.w, intermetallic compound with Al.
These considerations are illustrated by FIG. 1, showing a hypothetical ternary phase diagram for the Mg--Al--Me system (Me being the unspecified third alloying element). Let us assume that in this system can form in general, three intermetallic compounds: Mg.sub.17 Al.sub.12, Mg.sub.x Me.sub.y, Al.sub.z Me.sub.w. In order to suppress the eutectic reaction involving the formation of the .beta.-phase compound Mg.sub.17 Al.sub.12, the element Me will have to react with aluminum to form the intermetallic compound Al.sub.2 Me.sub.w. In this case the pseudobinary section Mg--Al.sub.z Me.sub.w will be active. This will take place only in the case when the affinity of Me to Al is higher than that of Mg and the formation of Al.sub.z Me.sub.w is preferential to the formation of the Mg.sub.x Me.sub.y intermetallic compound.
The analysis of available binary phase diagrams Mg--Me and Al--Me have shown that only the following elements can comply with the requirements mentioned above:
rare earth elements (Ce, La, Nd, etc.)
alkaline earth elements(Ca, Ba, Sr)
3d--transition elements (Mn, Ti).
Calcium would seem to be the most attractive as the main additional alloying element, due to its low cost and to the presence of suitable master alloys with low melting points on the market. In addition, the low atomic weight of calcium compared with rare earth elements permits lower addition by weight in order to obtain the same volume percentage of the Al.sub.z Me.sub.w. strengthening phase.
The addition of Ca to Mg--Al--Mn and Mg--Al--Zn alloys is disclosed in some prior art patents. Thus, German Patent Specification No. 847,992 discloses magnesium based alloys which comprise 2 to 10 wt % aluminum, 0 to 4 wt % zinc, 0.001 to 0.5 wt % manganese, 0.5 to 3 wt % calcium and up to 0.005 wt % beryllium. A further necessary component in these alloys is 0.01 to 0.3 wt percent iron. PCT Patent Specification WO/CA96/25529also discloses a magnesium based alloy containing 2 to 6 wt % aluminum and 0.1 to 0.8 wt % calcium. The essential feature of that alloy is the presence of the intermetallic compound Al.sub.2 Ca at the grain boundaries of the magnesium crystals. The alloy according to that invention may have a creep extension of less than 0.5% under an applied stress of 35 MPa at 150.degree. C. during 200 hours.
British Patent Specification No. 2296256 describes a magnesium based alloy containing 1.5 to 10 wt % aluminum, less than 2 wt % rare earth elements, 0.25 to 5.5 wt % calcium. As optional components this alloy may also comprise 0.2 to 2.5 wt % copper and/or zinc.
Magnesium alloying with Zn are commonly used for solid solution strengthening of the matrix and decreasing the sensitivity of Mg alloys to corrosion due to heavy metal impurities. Alloying with Zn can provide the required fluidity and hence much lower Al levels may be used. Magnesium alloys containing up to 10% aluminum and less than about 2% Zn are die castable. However, a higher concentration of Zn leads to hot cracking and microporosity problems.
U.S. Pat. No. 3,892,565 discloses that at still higher Zn concentrations from 5 to 20%, the magnesium alloy again is easily die castable. As confirmation for this, U.S. Pat. No. 5,551,996 also describes a die castable magnesium alloy containing from 6 to 12% Zn and 6 to 12% Al. However, these alloys exhibit considerably less creep resistance than commercial AE42 alloy.
PCT patent application WO/KR97/40201 discloses a magnesium alloy for high pressure die casting, comprising 5.3 to 10 wt % Al, 0.7 to 6.0 wt % Zn, 0.5 to 5 wt % Si, and 0.15 to 10 wt % Ca. The authors claim that this alloy is die castable and exhibits high strength, toughness and elongation. However, this application is not concerned with creep resistance.
It is an object of this invention to provide magnesium alloys suitable for elevated temperature applications.
It is another object of this invention to provide alloys which are particularly well adapted for use in the die casting process.
It is a further object of this invention to provide alloys which may also be used for other applications such as sand casting and permanent mould casting.
It is a still further object of this invention to provide alloys which have high creep resistance and exhibit low creep deformation.
It is a still further object of this invention to provide alloys which have low susceptibility to hot tearing.
It is a still further object of this invention to provide alloys which have the aforesaid properties and have a relatively low cost.
Other objects and advantages of this invention will appear as the description proceeds.